Energy Dynamics: Biology Lab

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  • 0:02 Energy in Biological Systems
  • 1:38 Energy Flow Through Plants
  • 4:11 Energy Flow in Plants…
  • 8:15 Lesson Summary
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Lesson Transcript
Instructor: Jennifer Szymanski

Jen has taught biology and related fields to students from Kindergarten to University. She has a Master's Degree in Physiology.

Even young science students know that energy flows through an ecosystem, from the sun to producers to consumers. But what happens, exactly, to that energy? This laboratory explores ecosystem energy dynamics by creating a very simple food chain.

Energy in Biological Systems

Somewhere in your science studies, you probably learned that all energy in living organisms is derived from a single source: the sun. The sun's energy is changed into chemical energy by plants. This energy is transferred into animals that eat the plants, and then goes into other animals that eat those animals and so forth, in what we know as a food chain.

Well, it's time to step up your ecosystem energy game a little, because it's just not quite that simple. Let's begin by brushing up on some vocabulary. Producers, you will recall, are organisms that change light energy (or sometimes inorganic chemicals) into usable chemical energy, usually in the form of the simple sugar glucose. Ecologists say that the total energy made by producers in an ecosystem in a given period of time is called gross primary productivity, or GPP.

Plants use some of this energy to sustain life in cellular respiration. The rest they use to create more cells (also known as growing) and to reproduce. The energy the plants 'put back' into the ecosystem by creating more cells and tissues is called is called net primary productivity, or NPP. Therefore, GPP - R (energy used in respiration) = NPP.

You can think of NPP as the amount of energy in an ecosystem that's available to be, for example, eaten by an animal. This energy stored in cells and tissues is also called biomass.

Investigation: Energy Flow Through Plants

We can track the flow of energy (measured in kilocalories, or kcal) through an ecosystem by estimating its NPP. Notice the deliberate use of the word 'estimating.' This is important for many reasons. First, as energy flows between organisms in an ecosystem, some is lost to the environment (mostly as heat). This loss can be hard to measure accurately.

Second, ecologists estimate the productivity of an ecosystem by measuring the change in plant biomass over time. To get a true idea of how much energy is in every gram of plant matter, you'd rid your plants of all their water, which doesn't provide any energy. Unfortunately, that would also kill the plants, so you wouldn't be able to take the second measurement needed to correctly calculate the change in biomass. Third, energy comes from different molecules, and these molecules hold different amounts of energy. Fats, for example, store nine calories/g, while carbs store about four. So, a plant that has more fats in its cells would hold more energy than a plant that has more starches.

Energy flow through plants in an ecosystem

Let's review how we could, in theory, create an example ecosystem. We'll start by estimating the energy flow through a plant. For ease, we'll choose as our example plant brassica rapa, a member of the cabbage family. How does energy flow through the plant? Light energy is absorbed by the plant and, ultimately, is released as waste, used for respiration or goes into biomass.

If we have a population of about 50 plants, all brassica rapa, all planted at the same time, and all grown under the same conditions, we can estimate the biomass of a plant by drying ten plants at day seven, then weighing them once most of the water has been removed. We can use our data to make a table like this. Note that the value of 4.35 kcal is a standard value derived by scientifically determining how much energy is present in brassica plants.

Age of plant Initial mass (10 plants) Final mass (10 plants) % of plant made of biomass Energy available (g biomass x 4.35 kcal/10 plants) NPP/plant/day
7 20 g 4 g 20% 17.4 0.25

Then we'll repeat the weigh-in/drying/weigh in process again at 14 days. What changes do you expect to see? We will most likely see an increase in both wet and dry mass, since the plants are growing. We'll also see an increase in the energy and NPP per day/per plant, since the plants have increased the energy available in their ecosystem by adding new cells and tissues.

Investigation: Energy Flow Through Plants and Animals

Now we'll complicate things a little bit by adding an animal into the mix. Primary producers are eaten by primary consumers, or animals that eat plant matter as part of their diet. Our producer in this part of the experiment will be a Brussels sprout, which is a member of the same family as the plant we used in the last part of the experiment.

First, we'll take a large Brussels sprout and weigh it, then cut it in half. Next, we'll weigh ten cabbage butterfly larvae, and calculate the average weight of one organism. Then we'll put them in an enclosure, stand back, and let them do their thing. Every day, we'll have to take into account the waste the larvae leave behind by gathering and weighing it daily.

At the end of three days, the results might look something like this:

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